Il ruolo dell'allocazione del carbonio nell'acclimatamento e nella risposta agli stress abiotici delle piante

Plants are sessile organisms and have develop different physiological strategies to cope with abiotic stresses. In the view of ongoing climate changes is important to delve into plants response to stress to forecast future dynamics. This thesis aimed at investigating on the plants short-term physiological response and acclimation to new climatic conditions. It focused on plant growing in three vulnerable environments to increase in stress intensity due to climate changes: alpine tundra (snow cover), mediterranean basin (drought) and wetlands (waterlogging). Non-structural carbohydrates (NSC) concentration was chosen and analysed as a pivotal trait involved in stress response and to investigate the management of carbon in different plant organs. Beyond soluble NSC and starch also the different pool of sugars (i.e. glucose, fructose, maltose and sucrose) were measured to provide a complete overview in drought response. In addition, polyols and organic acids were considered in the analysis since they are compounds directly related to carbohydrates metabolism. Carbon compounds were correlated with growth traits and hydraulic parameters, given their close relationship when considering the entire plant metabolism. Analysis was performed on different plant organs, including leaves, stem divided into bark and wood, and roots. NSC have been confirmed a key trait to depict plant physiological responses in all the environments considered. In alpine tundra, relationships between NSC and growth have shown how common junipers are plants able to overcome the risk of early snow melt and exploit a longer photosynthetic period to support both growth and NSC accumulation. Plants growing in dry environments have to cope with water deficit. This thesis showed as sugars accumulation was allowed at the expense of plant growth to provide osmolytes necessary to maintain cells turgor. It was also demonstrated that a greater sugars accumulation was promoted by a previous mild drought, suggesting a priming effect. In this context, beyond providing extra carbon gain with the stem photosynthesis, bark has proven to be main site for sugars store in stem. The advantage is that sugars can be mobilized from bark both via phloem transport and loading through parenchymatic rays into xylem sap. Working on Fraxinus ornus, we observed that the pool of sucrose has been the most involved in drought response, supporting the osmotic adjustment and providing energy at sustaining the recovery machine. Starch degradation sustained mannitol accumulation during drought, reinforcing the osmotic function, especially in sucrose limitation conditions due to the absence of stem photosynthesis. NSC metabolism during abiotic stress include also malic acid produced through glycolysis and Krebs cycle. Under waterlogging oxygen consumption of both roots and mitochondria was prevented by lack of malic acid because extruded from roots to waterlogged soil to buffer protons responsible for toxicity given by dissociation of short organic acids, produced in anoxia conditions. Eventually, this thesis highlighted that the development of physiological mechanisms is dependent on both plant adaptation and acclimation. A same species can be characterized by different ecotypes that, on the basis of phenotypic variability, can express different hydraulic behaviour and consequently a different allocation of carbon, shaping acclimation capacity to different climate conditions. In conclusion this thesis disentangles some interesting points in the role of NSC in the response of abiotic stresses. Some key point that should be considered in future investigations are highlighted for a full comprehension of this complex scenario in the expect of improving plant ecosystem protection and restoration in the ongoing scenario of climate changes.